1. Field of the Invention
This invention relates to optical sensors designed to detect liquid pools within the soil. More specifically, this invention relates to an apparatus designed to detect and discriminate between liquid pools of dense non-aqueous phase liquids (DNAPL) such as chloroform, trichloroethylene (TCE), and carbon tetrachloride and ground water or soil gases.
2. Description of the Related Art
Currently, numerous governmental agencies and private organizations are involved in massive programs designed to identify, characterize, and remediate various hazardous waste sites around the country. A major problem at many sites is the contamination of the soil by dense non-aqueous phase liquids (DNAPL) such as chloroform, trichloroethylene (TCE), and carbon tetrachloride. These materials, which may have been dumped into settling ponds or may have leaked from buried containers, slowly sink into the soil and, at some point, reach an impermeable layer such as a clay barrier. Over time, these materials collect in a layer or pool over the barrier. Additionally, many of these materials vaporize and contaminate the soil above the pool. Therefore, many of the materials in the soil can exist in two distinct phases, liquid and gas.
A sensor which is able to locate liquid pools of DNAPL such as TCE and identify the type of liquid would be very useful for environmental applications. For example, such a sensor could be used to locate and identify underground pools such that a three dimensional diagram of the contaminated area can be created. These diagrams are useful in identifying the contaminated area and creating and implementing a comprehensive clean-up plan.
Currently, liquid pools of hazardous materials are located by drilling of one or more test wells to obtain samples which are then analyzed. However, this method has several limitations. Because the test wells often must be several hundred feet deep, this process takes time and is quite expensive. Additionally, because the materials can exist in two distinct phases, the samples often contain soil that has been contaminated by vapors. This creates a problem of having to treat the samples before they can be disposed. Furthermore, a three dimensional map of the contaminated area cannot be obtained with this method.
While several fiber optic sensors and optical probes have been developed to detect liquid levels as well as determine the type of material present, such conventional probes suffer from one or more disadvantages. For example, sensors which identify liquid levels do not accurately identify the type of liquid present. Sensors which do identify the type of material present generally require that the fiber be in contact with the material.
This requirement limits the usefulness of the sensor for environmental applications such as the detection of liquids within the soil because the fibers are too fragile to be repeatedly driven into the soil. Additionally, many sensors which identify the type of material do not differentiate between the liquid and gas phase of the material. Furthermore, many of these sensors require expensive equipment and/or a relatively high degree of skill to operate.